Prolog programming: a do-it-yourself course for beginners Day 1 Kristina Striegnitz Department of Computational Linguistics Saarland University, Saarbr¨ ucken, Germany kris@coli.uni-sb.de http://www.coli.uni-sb.de/˜kris Day 1: Facts, Rules, and Queries – p.1
What is this course about? This course • introduces the basic concepts of Prolog programming, • gets you started on programming yourself, and • shows you how to do some natural language processing using Prolog. Day 1: Facts, Rules, and Queries – p.2
Overview Day 1: Prolog as a system for specifying and querying knowledge bases Day 2: how Prolog finds answers to queries Day 3: an important data structure: lists Day 4: specifying grammars in Prolog Day 5: building parsers in Prolog Day 1: Facts, Rules, and Queries – p.3
Organization • Each session has a lecture part and a practical part (more about the practical part later). • The reader contains the first eight chapters of Learn Prolog Now! by Blackburn, Bos, and Striegnitz. It also contains some questions (with answers) for each day, if you want to review what we have done. • The slides are available at the course homepage http://www.coli.uni-sb.de/˜kris/esslli04prolog . • Questions during class: Ask any time and as many as possible. • Questions outside class: You can find me near the ESSLLI desk from 10:30 to 11:30. Day 1: Facts, Rules, and Queries – p.4
Day 1: Facts, Rules, and Queries Today: How to specify knowledge bases in Prolog and how to query them. Reader: Lecture 1 of Learn Prolog Now! Day 1: Facts, Rules, and Queries – p.5
Prolog as a system for querying knowledge bases Today Knowledge Tomorrow Base Prolog Interpreter Query Answer Day 1: Facts, Rules, and Queries – p.6
kb1: A knowledge base of facts wizard(harry). wizard(ron). wizard(hermione). muggle(uncle_vernon). muggle(aunt_petunia). chases(crookshanks, scabbars). Day 1: Facts, Rules, and Queries – p.7
kb1: queries we can ask wizard(harry). wizard(ron). ?- wizard(harry). wizard(hermione). yes muggle(uncle vernon). muggle(aunt petunia). ?- chases(crookshanks,scabbars). chases(crookshanks,scabbars). yes ?- muggle(harry). no ?- muggle(dumbledore). no ?- wizard(dumbledore). no ?- witch(hermione). ERROR: Undefined procedure: witch/1 Day 1: Facts, Rules, and Queries – p.8
kb1: more queries we can ask wizard(harry). wizard(ron). ?- muggle(X). wizard(hermione). X = uncle vernon ; muggle(uncle vernon). muggle(aunt petunia). X = aunt petunia ; chases(crookshanks,scabbars). no ?- chases(X,Y). X = crookshanks Y = scabbars ; no ?- chases(X,X). no Day 1: Facts, Rules, and Queries – p.9
A bit of syntax: atoms and variables Atoms: • All terms that consist of letters, numbers, and the underscore and start with a non-capital letter are atoms : harry , uncle vernon , ritaSkeeter , nimbus2000 , . . . . • All terms that are enclosed in single quotes are atoms : ’Professor Dumbledore’ , ’(@ *+ ’ , . . . . • Certain special symbols are also atoms: + , , , . . . . Variables: • All terms that consist of letters, numbers, and the underscore and start with a capital letter or an underscore are variables : X , Hermione , ron , . . . . is an anonymous variable: two occurrences of are different • variables. Day 1: Facts, Rules, and Queries – p.10
A bit of syntax: complex terms Complex terms: • Complex terms are of the form: functor ( argument , ..., argument ) . • Functors have to be atoms. • Arguments can be any kind of Prolog term, e.g., complex terms. likes(ron,hermione) , likes(harry,X) , f(a,b,g(h(a)),c) , . . . . Day 1: Facts, Rules, and Queries – p.11
A bit of syntax: facts and queries • Facts are complext terms which are followed by a full stop. wizard(hermione). muggle(uncle vernon). chases(crookshanks,scabbars). • Queries are also complext terms which are followed by a full stop. ?- wizard(hermione). Query Prompt provided by the Prolog interpreter. Day 1: Facts, Rules, and Queries – p.12
kb2: a knowledge base of facts and rules eating(dudley). happy(aunt petunia) :- happy(dudley). happy(uncle vernon) :- happy(dudley), unhappy(harry). happy(dudley) :- kicking(dudley,harry). happy(dudley) :- eating(dudley). Day 1: Facts, Rules, and Queries – p.13
kb2: a knowledge base of facts and rules eating(dudley). happy(aunt petunia) :- happy(dudley). happy(uncle vernon) :- happy(dudley), unhappy(harry). happy(dudley) :- kicking(dudley,harry). happy(dudley) :- eating(dudley). if ... then ...: If happy(dudley) is true, then happy(aunt petunia) is true. Day 1: Facts, Rules, and Queries – p.14
kb2: a knowledge base of facts and rules eating(dudley). happy(aunt petunia) :- happy(dudley). happy(uncle vernon) :- happy(dudley) , unhappy(harry). happy(dudley) :- kicking(dudley,harry). happy(dudley) :- eating(dudley). and: If happy(dudley) is true and unhappy(harry) is true, then happy(uncle vernon) is true. Day 1: Facts, Rules, and Queries – p.15
kb2: a knowledge base of facts and rules eating(dudley). happy(aunt petunia) :- happy(dudley). happy(uncle vernon) :- happy(dudley), unhappy(harry). happy(dudley) :- kicking(dudley,harry). happy(dudley) :- eating(dudley). or: If kicking(dudley,harry) is true or if eating(dudley) is true, then happy(dudley) is true. Day 1: Facts, Rules, and Queries – p.16
Querying kb2 eating(dudley). happy(aunt petunia) :- happy(dudley). ?- happy(dudley). happy(uncle vernon) :- happy(dudley), yes unhappy(harry). ?- happy(aunt petunia). happy(dudley) :- kicking(dudley,harry). yes happy(dudley) :- eating(dudley). ?- happy(uncle vernon). no ?- happy(X). X = aunt petunia ; X = dudley ; no Day 1: Facts, Rules, and Queries – p.17
A bit of syntax: rules • Rules are of the form Head :- Body . • Like facts and queries, they have to be followed by a full stop. • Head is a complex term. • Body is complex term or a sequence of complex terms separated by commas. happy(aunt petunia) :- happy(dudley). happy(uncle vernon) :- happy(dudley), unhappy(harry). Day 1: Facts, Rules, and Queries – p.18
kb3: facts and rules containing variables This knowledge base defi nes 3 predi- father(albert,james). cates: father/2 , mother/2 , and father(james,harry). wizard/1 . mother(ruth,james). mother(lili,harry). wizard(lili). wizard(ruth). For all X , Y , Z , if father(Y,X) wizard(albert). and wizard(Y) is true is true mother(Z,X) and is true wizard(X) :- father(Y,X), wizard(Z) and is true, then wizard(Y), wizard(X) is true. I.e., for all X , mother(Z,X), if X ’s father and mother are wizards, wizard(Z). then X is a wizard. Day 1: Facts, Rules, and Queries – p.19
father(albert,james). Querying kb3 father(james,harry). mother(ruth,james). ?- wizard(james). mother(lili,harry). yes wizard(lili). ?- wizard(harry). wizard(ruth). yes wizard(albert). ?- wizard(X). wizard(X) :- father(Y,X), X = lili ; wizard(Y), X = ruth ; mother(Z,X), X = albert ; wizard(Z). X = james ; X = harry ; no ?- wizard(X), mother(Y,X), wizard(Y). X = james Y = ruth ; X = harry Y = lili ; no Day 1: Facts, Rules, and Queries – p.20
Prolog terms (overview) atoms: Start with non-capital letters or are enclosed in single quotes. harry , nimbus2000 , ’Professor Dumbledore’ , aunt petunia numbers 3, 6, 2957, 8.34, ... variables Start with a capital letter or an underscore. Harry , harry complex terms An atom (the functor) is followed by a comma separated sequence of Prolog terms enclosed in parenthesis (the arguments). like(harry, X) , np(det(the),n(potion)) Day 1: Facts, Rules, and Queries – p.21
Practical Session • Go to the course homepage: http://www.coli.uni-sb.de/˜kris/esslli04prolog . • There you find links to the material for the practical sessions. • Today’s practical session starts with an explanation of how to use the Prolog interpreter. • All exercises come with a page of hints and a solution. But don’t peek before you have really tried to solve it yourself! • You don’t have to do all exercises. Decide for yourself whether you want to stick to the basic ones for the moment or whether you want to skip some of them to also have a look at the more advanced ones. • If you need help, yell! If I am busy, ask your neighbors. Day 1: Facts, Rules, and Queries – p.22
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